Staszek indeed. Moving tags in the tag tree has no impact on which tags a discussion possess.
The only thing that impacts existing discussions is if you rename a tag (name, icon, slug). For example if "Meetings" becomes "Meeting Archives 2019" (via renaming), then all discussions that had the tag will reflect the tag rename. Then you could create a new "Meeting" tag that uses the same tag slug and ordering as the old tag. So people will actually post new discussions under a new tag, but they won't notice any difference if you name it the same way as before.
To actually change the tag a discussion has, you'd need an extension. And currently I don't think we have any extension to mass-update tags on discussions, or to automate adding parent tags when moving tags in the tag tree.
